Temperature compensated bore hole radioactivity apparatus



2,709,753- TEMPERATURE COMPENSATED BORE HOLE RADIOACTIVITY APPARATUSOriginal Filed Oct. 1,, 1941 y 1955 s. KRASNOW ETAL 4 Sheets-Sheet 1INVENTORS. SHELLEY" KRASNOW I MgYER JOSEPH TEST THE! ATTORNEYS.

May 31, 1955 s. KRASNOW ETAL TEMPERATURE COMPENSATED BORE HOLERADIOACTIVITY APPARATUS Original Filed-Oct. 1, 1941 4 Sheets-Sheet 2F|G.5A.

INVENTORS. SHELLLEY KRASNOW MEYER JOSEPH TEST -Qfmu-q THEIR ATTORNEYS.

y 31 1955 5. KRASNGW E ,1 ,153

TEMPERATURE COMPENSATED BORE HOLE RADIQACTIVITY APPARATUS Original FiledOct. 1, 1941 4 Sheets-Sheet s IIIIIIIIIIIIIIIA will! INVENTORS, E YKRASNOW 5/ Y JOSEPH TEST THELR ATTORNEYS.

May 31, 1955 s. KRASNOW ETAL 2,709,753

TEMPERATURE COMPENSATED BORE HOLE RADIOACTIVITY APPARATUS Original FiledOct. 1, 1941 4 Sheets-Sheet 4 I INVENTORS. SHELLEY KRASNOW MEYER JOSEPHTEST THEIR ATTORNEYS.

United States Patent "ice 2,709,753 TEMPERATURE COMPENSATED BORE HOLERADIOACTIVITY APPARATUS Shelley Krasnow, Fairfax, Va., and Meyer JosephTest,

Kansas City, Mo., assignors to Schlumberger Well Surveying Corporation,Houston, Tex., a corporation of Delaware Original application October 1,1941, Serial No. 413,240. Divided and this application January 2, 1951,Serial No. 203,814

Claims. (Cl. 259-336) This invention relates to apparatus for measuringradioactivity in bore holes. This subject has been taught inconsiderable detail in the earlier work of one of the co-inventors. Thepresent apparatus is intended to provide improved means for theexpeditious and accurate measurement of radioactivity in deep boreholes, where high temperatures also exist.

This application is a division of our copending application Serial No.27,857, filed May 19, 1948, now Patent No. 2,640,161, for EnergizingSystems for Bore Hole Radioactivity Apparatus, which in turn is adivision of our parent application Serial No. 413,240, filed October 1,1941, for Borehole Radioactive Apparatus, now abandoned. Portions of thesubject matter disclosed in the latter application are now being claimedin the said application Serial No. 27,857, and in our copendingdivisional application Serial No. 27,856, filed May 19, l9 i8, nowPatent No. 2,605,435, for Construction of Geiger Muller Tube. Thepresent application is addressed to other subject matter disclosed inthe parent application which involves means for minimizing the effectsof variations in bore hole temperature upon bore hole radioactivityapparatus.

it is an object of the invention to provide a convenient radioactivebore hole apparatus, capable of making measurements of radioactivity ata desired depth in a bore hole.

It is a further object of the invention to provide such an apparatuswhich will record continuously the radioactivity measured at ditlerentdepths.

it is a further object of the invention to provide an apparatus whichwill record faithfully and will integrate pulses from a counter deviceoperated in the bore hole.

it is a further object of the invention to obtain a heightened result inthe measurement of radioactivity.

it is a further object of the invention to provide a compact counterelement and associated circuit which may be contained within a narrowcartridge capable of being lowered into a bore hole.

it is a further object of the invention to provide radioactive bore holeapparatus which may be easily assembled and disassembled, and which isprotected from mechanical shocks when in use.

it is a further object of the invention to provide a radioactive borehole apparatus which does not require a primary voltage source withinthe cartridge.

It is a further object of the invention to provide means for usingalternating current generated at the surface of the ground to providehigh voltage direct current operating a radioactive measuring instrumentdeep in a bore hole.

It is a further object of the invention to provide an improved systemfor amplifying and transmitting pulses produced by a radioactivemeasuring system utilized in a bore hole.

Other objects and advantages of the invention will be apparent from thefollowing drawings, in which:

Fig. 1 represents a schematic view of the apparatus as used in a borehole;

Fig. 2 represents a longitudinal cross-sectional view of Patented May31, 1955 the cartridge shown in Fig. 1, showing the relative arrangementof the elements;

Fig. 3 shows a detail indicating the manner of mounting the circuitelement in the embodiment shown in Fig. 2;

Fig. 3A is a schematic diagram of a typical thermostatic system forcontrolling the electrical energy fed to a heating coil in the apparatusshown in Fig. 2;

Fig. 4 shows an alternate system, adapted for use where temperaturechanges affect elements of the system;

Fig. 5 shows alongitudinal cross-sectional view of the counter tube;

Fig. 5A shows a counter tube of the type illustrated in Fig. 5 but witha double wall for thermal insulation;

Fig. 6 shows the circuit diagram of the apparatus, indicating wiring ofthe parts within the cartridge;

Fig. 7 shows the wiring diagram of that portion of the apparatus at thesurface of the earth;

Fig. 8 shows an enlarged view of the insulator and seal, showing themethod of assembly; and

Fig. 9 shows a plan view of one of the sections shown in Figs. 3 and 4.

1 represents a bore hole drilled in the earth in which may or may notexist a metallic casing 2. The bore hole may or may not be filled withliquid 3. Suspended within the bore hole is an element 4, serving tocontain certain of the elements of the apparatus and to place theseelements proximate to the strata from which it is desired to obtainindications. Suspending the element 4 'is a multiconductor cable 5,which serves to position the element 2-, to conduct energy into the saidelement and also to receive responses therefrom. The cable 5 passes overa measuring wheel 6, which at all times indicates the depth of theelement 4, and which may be connected to other portions of the apparatusto record the depth against any other desired quantity. The wheel 6rests upon a support '7 at the top of the bore hole. Cable 5 may bereeled or unreeled by means of a drum 8, which has slip rings allowingconnection to be made during rotation and for any position of the drumto a number of stationary wires 9. These wires lead to apparatus 10 andii, whose purpose will be hereinafter described.

Referring now particularly to Fig. 2, 12 represents the exterior of thecartridge 4. This is preferably made of a strong metal such as highstrength steel, and may be provided with a corrugated exterior surface13 where the rays are to pass through. At either end of the member 12internally threaded portions 14 and 15 are provided into which suitablesupports and closures may be fastened. At the lower end of the cartridgeis a closure 16 which may be screwed into the tube 12 and which bearsagainst a gasket represented schematically as 17 to shield the apparatusagainst high hydrostatic pressures. Above the upper portion of theelement 16 is a threaded insert 18 which serves as a mechanical support.This bears against a coil spring 19, which in turn bears against aninsulating block 29. Fastened rigidly to the insulating block 26 is aradioactive sensitive element 21. Insulating ring 22 is fastened rigidlyto the upper end of the radioactive sensitive element, and a preferablymetallic tube 23 rests upon the upper end of this insulator. Anadditional threaded insert 13a is placed above tube 23, and serves tocompress a coil spring 19. This assemblage maintains the element 21 in asort of floating support, held by springs above and below. The elements2:) and 22 will make light frictional contact against the walls of thetube 12, which will serve to damp mechanical oscillations. This may beaided by providing small springs which bear against the interior wall ofthe tube 12 and thus provide definite frictional contact. An upperclosure 24 is provided which compresses a gasket 25 and serves tomaintain the entire assemblage in fluid-tight and pressureresistantcondition. Wires lead through upper member 18a, through pressure-tightinsulating bushings 26 and 27, respectively, and further to the surfaceof the earth. These wires are attached in mechanically strong fashion toelement 24 so that they may be used to support the entire apparatus.Lying above insulating ring 22 is assemblage 23, containing theauxiliary elements and other portions required for operation. This is inthe form of a relatively rigid frame upon which are mounted the vacuumtubes, condensers and resistors necessary for oporation. The frame 23rests upon springs, which may be three in number and is also helddownward by springs 30, which may also be three in number.

The element 21 employs improved construction, which has advantages foruse in measuring radioactivity in bore holes. This consists of ametallic tube 31 (Fig. 5) which may be provided with threaded ends 32,32. Fastened to these threaded ends are internally threaded collars 33,into which are screwed externally threaded inserts 34 and 35. Bothinserts have cap screws or the equivalent, 36, threaded therein andserving to bear against heads 37 and 38, respectively. Head 37 has ablind cavity 39 into which is fitted an insulating tube dil, held bysome means such as a packing of lead ii. A metallic wire 42 extends fromtube dll and is firmly connected thereto. An opening 43 in head 37 has atube, preferably a thin copper tube, fastened therein as by soldering.This passes through an opening in head 35 and may be connected to asuitable apparatus for pumping and filling the tube. A closure in theform of a well constructed needle valve 44 may be provided at theterminal portion of the tube 45. Alternatively, the tube may be pinchedclosed after evacuation or filling with a pair of blunt cutting nippers,or soldered shut. Head 38 has a neck portion 46 through which isprovided an opening 47. Soldered into this opening is a thin metallictube 48 (Fig. 8), which may be of copper, and which extends beyond theexterior of the head 38. At the terminal portion 4 of this tube isfastened an insulating tube 50 which may be a glass or quartz tube whichis sealed to the copper by means of either soldering by standardmethods, or by the use of a so-called Housekeeper seal. The insulatingtube 5i) extends inwardly forming a projecting portion 51. This servesas an electrostatic shield which prevents spurious effects in theapparatus. The wire 42 passes centrally through tube 50 and a gastightseal is formed at 52a. The wire preferably has at its end a. metal ofthe type of tungsten 52 where the seal is to be formed.

The assemblage 23 is preferably made in the form a unitary structurewhich can be inserted into the cartridge 12 or removed conveniently atwill.

The assemblage 23 (Fig. 3) may be made of a number of plates such as 54,which are held in rigid spaced relation by means of rods 5s, andseparators 59. Nuts 53 serve to clamp the entire assemblage together.The nrunber of rods 56 may be two, three, four, or more in number. Eachof the elements 54 may be in the form of a of metal provided, forexample, with vacuum tube sockets such as 69a (Fig. 9), and openingssuch as 510, through which wired connections may be taken from one levelinto another. Circuit elements such as condensers representedschematically as 5'7 (Fig. 3) may be fastened to a plate such as 53. Theentire assemblage is wired together so that only the input and outputwires extend below and above respectively.

It has been found that certain elements such as condensers will havetheir capacity vary in proportion to the temperature. This willintroduce erroneous results in some cases in the apparatus, and wheresuch conditions exist the structure shown in Fig. 4 may be utilized.Here a frame 63, similar to frame 28 is utilized, a Dewar fiat; 6% beingmounted on cushioned supports and held rigidly with respect to the restof the apparatus. A plug 61 of thermal insulating material such as corkis inserted into the lower part of this flask. A circuit element such ascondenser 62 may be mounted attached to this element. The assemblagewill, therefore, serve to hold these elements at relatively constanttemperature, and in any event to cause the changes of temperature to bevery gradual.

A detail of the spring mounting of the assemblage 28 is shown in Fig. 3.Here threaded rods 56 pass through a disk 64, which is attached rigidlyto the top of counter tube 21 by means of screws 65. At the lower end ofthe rod 56, a shoulder portion 68 is provided. Underneath this aresprings 660, which may be of coil form. Underneath disk 64 an additionalset of springs is provided. Nuts are fastened at the lower portion ofrod 56, thus providing a floating spring mounting for the entireassemblage 28.

Any other portion of the apparatus which is found to be temperaturesensitive may be mounted inside the Dewar flask and so kept atrelatively constant temperature. Where the use of a Dewar flask isobjectionable, as with severe jarring which might break such a flask, athermostatic system employing a heater coil operated from the inputvoltage and controlled by thermally operated relays may be utilized. Thetemperature for which the thermal system would be set would be the hihest one expects to encounter. Regardless of the exterior temperaturethen, the apparatus would always operate at constant temperature. Theheater coil 30% in such a case would best be placed in the gap betweenthe insulator 2t) and insert 18 (Fig. 2), suitable openings 301 and 3&2being provided in members 20 and 22, respectively, to allow freecirculation of air to maintain a substantially constant temperature. Theheater coil 390 would have to be of large capacity, since when immersedin cold liquid, the heat loss from the cartridge would be appreciable.This heat loss may be reduced considerably by lining the entirecartridge 12 with a thin heat insulator 3%. This will have relativelylittle stopping power for the gamma rays.

The thermostatic system for energizing the heater coil from the inputvoltage under the control of thermally operated relays may be of ti etype shown in Fig. 3A. Here the input voltage is supplied to the primarywinding 3%: of a transformer 1W5 the secondary winding 3&6 of which isconnected to the heater coil 3%. The trans former secondary winding 3%is provided with a plurality of taps 397 and Bil-S, only two being shownin Fig. 3/\, although either more or less than two may be used.Connected between one terminal of the transformer secondary winding 3G6and the tap 397 is a temperature respon sive relay 39). Anothertemperature responsive relay 31th is connected between the taps 397 andand a third temperature responsive relay 311 is connected bctween thetap 398 and the other terminal of the secondary winding 3%.

The temperature ipsponsive relays 3&9, 31b and 311 are preferablydesigned so that when they are exposed to the temperature at thesurface, their contacts are open so that the full voltage of thetransformer secondary winding 3&6 is fed to the heater coil Silt), thisvoltage being sufficient to maintain the temperature in the body 4 atthe highest temperature expected in the bore hole. The relays 369, 316and 311 further are adjusted so as to close their contacts forsuccessive increments in bore hole temperature in that order. As eachrelay closes, it short circuits a portion of the transformer secondarywinding 3%, thus reducing the voltage led to the heater coil. Thereduction in voltage is sufficient to maintain the temperature withinthe body at the value corresponding to the highest temperature expectedin the bore hole.

The Geiger Muller tube itself, in many cases, will be found to besensitive to temperature and to require different operating voltages atdifferent temperatures. The

Geiger Muller tube may have an outer wall 31 made double, with anannular cylindrical space 312 which is evacuated, as shown in Fig. 5A.Thus, the tube will be thermally insulated against external temperaturechange.

It will be noted that the spring mountings for the counter tube 2.1 andthe assemblage 28 are individual. The tube structures should preferablyhave different periods of oscillation so as to tend to suppressoscillations. it is understood that the unitary assemblage 28, may holdthe elements of any desired circuit or auxiliary apparatus to operatethe radioactive sensitive element, and to receive responses therefrom.

Referring now particularly to Fig. 6, 69 and 79 represent the inputpower lead of the apparatus, 6? being a grounded negative lead, and illbeing positive lead. The voltage between these two may be 221' and 256volts or any other desired value. This voltage is applied to thefilaments of the various vacuum tubes 73, 73, i4 and 75, throughresistors il and 'iln'. This reduces the voltage to the proper value forall the filaments appearing. The voltage across resistor 71a is alsoapplied to the plates of the various vacuum tubes as shown. Anoscillatory circuit is provided comprised chiefly of vacuum tube 72,transformer "73, condenser 79, and condensers 13% and 131. Thisgenerates a high voltage by oscillation at a suitable frequency whichmay be 200 ltilocycles. This voltage is applied across the rectifiertube 75, which furnishes a high direct current volta e through resistor81 and across Geiger Muller counter tube 21. Condenser 82 serves toimprove the filtering of the direct current voltage furnished to theGeiger Muller tube. The pulses produced by the breakdown or dischargeor" the Geiger Muller tube 21 are quenched by resistor Such pulses arefed to condenser 3 and across resistor placed between the filament andthe grid of amplifier 74. Tube 7 3 and resistance capacity coupledamplifier 73 both serve several functions. Tube 7d serves to amplify thepulses produced by the Geiger Muller tube 21, and to invert thesepulses. The original pulse is a negative one and is inverted to form apositive pulse. Tube 73 is operated as a Class C amplifier so that thesystem will be unresponsive to microphonics. It also serves to sharpenthe Geiger Muller tube pulses. As shown, the system will respond only toappreciable pulses of the size produced by the Geiger Muller tube. Thefinal amplified pulse is then led through wire 39 to the surface of theground. Condenser 79 serves to tune the secondary of the transformer 73so as to obtain a desired frequency.

The voltage output of the oscillator may be altered by detuning. Thismay be done by an automatically controlled condenser such as 7?, and thechange may be made responsive to temperature within the cartridge. Thus,a small capacity condenser with one plate made of bimetal, which willmove to or from the stationary plate, dependiru on temperature, may beutilized. The bimetal condenser may be of the type disclosed in priorPatent No. 1,862,931 to Gunn, for example. Thus, the voltage may bealtered automatically so as to be the proper value for operating theGeiger Muller tube, whose voltage characteristic sometimes changes withtemperature.

If desired, the resistor 129 may be made of sufiiciently high value tocause the oscillator to block intermittently, thus decreasing platecurrent of 72. This will produce a transient oscillation orsuper-regenerative oscillation in the coil 76.

It will be noted that the external voltage across the terminals 69 and7% will be added to the rectified voltage fed to the Geiger Muller tube21. This is of advantage in obviating the necessity of having theoscillatory circuit alone produce a high enough voltage to operate thecounter tube.

Fig. 7 shows the portions of the apparatus which are preferably keptabove ground. Here the terminals which are connected below ground arenumbered in the same fashion. It will be seen that the input or" thisapparatus,

which is the output of the apparatus below ground is represented by 39.This amplified pulse is, therefore, applied across resistor Std betweenterminals 7d and $9. Amplification is performed by resistance capacitycoupled amplifier tube 9 and one section of tube Amplification by tubeHi may be omitted, particularly if cable noises or other disturbancesare troublesome. The resistor 97 and condenser 93 act as a plate supplyfilter for the amplifier tube and prevent self-oscillation through platecoupling. Due to overloading of the tube 96, or the operation of thesaid tube at a suitable portion of its characteristic, all pulsesresulting from tube 96 will be of equal magnitude no matter what thesize of the pulse originally fed to the apparatus. This is desirablesince the number of pulses is the criterion of importance in a counterof this type, rather than the size of the pulses or the integratedaverage of the size of the pulses.

Where the output of the radioactive sensitive member is proportional toradioactivity, the tube will be oper ated at that portion of itscharacteristic which will give a value proportional to the input. Thus,if a proportional counter is used, or one in which the size of the pulseis dependent on the radioactive intensity, the tube 6 can be operated atthe approximately linear part of its characteristic so as to give anoutput proportional to the size of the input pulse.

The leveled pulses resulting from amplification by tube 96 are appliedthrough condenser 1th across the righthand section of the same tube,operating as a diode rectifier. This rectifier charges condenser .ldtl,which further charges condenser ltll, through resistor 325. The voltageacross condenser 101 is, therefore, proportional to the frequency of thepulses originally produced by the Geiger Muller tube. This voltage isapplied across a step ladde attenuator N32, with a switch 193, to selecta suitable tap thereof. The voltage output of the attenuator is appliedacross tube 104, which has a milliarnmeter in its plate circu t. Theplate current will be proportional to the grid voltage, and, therefore,will also be proportional to the number of pulses. The meter 195 will,therefore, read the integrated average of the pulses produced by theGeiger Muller counter.

In series with meter is an adjustable resistance 106, which is connectedin series with an external meter 107. The external meter may be arecording type while the meter is an ordinary indicator type. Theinstrument will, therefore, indicate and record simultaneously. The twometers may be placed at different localities. Alternatively meter 1&7may be replaced by any current sensitive or current responsive device.The attenuator 192 acts as a range device. Since any proportion of thevoltage appearing between the output terminals of the attenuator may beapplied across the vacuum tube 164, the range of the apparatus may bechanged as desired.

The input high direct current voltage is applied to terminals 112 and113 and is controlled by the switch 115'. A resistor 132 enables voltagefrom 112 and 113 to be stabilized by tubes 99 and 91 for applicationacross terminals 69 and 7b. The voltage regulators 92 and 93 which aregas discharge tubes connected in series, serve to supply accurate gridand plate voltages for the various tubes so as to assure accurate andstable operation thereof. The gas discharge voltage regulator tubes 99and 91 serve to supply a constant direct current voltage to be fedthrough terminals oh and 7b to the apparatus below ground.Alternatively, the voltage regulator tubes 90 and 91 may be mounted inthe cartridge 4. in order to set the meters 195 and N97 to zero, abucking circuit comprised by resistors ltlS and 169 is provided.Adjustable resistor 163 is used as the zero setting resistor.

The filament voltage for the tubes which are operated at the surface ofthe ground, is supplied preferably by a six volt or other suitablevoltage supply operated by switch 114, and furnished through supplyterminals 110 and 111.

spouses Because of the high temperatures often met with in deep boreholes, it will be found necessary when transformers or other similarelectrical elements are utilized, to have these units provided with aninsulating varnish capable of withstanding the temperatures encountered.The same remark applies to the resistors and condensers, although wherethese elements are maintained in a constant temperature enclosure aspointed out herein, such elaborate precautions are often unnecessary.

Various elements in the different circuits have been shown Without adescription being given of their exact function. The function of theseelements may be told from their relative positions in the respectivecircuits by those versed in the art.

A representative set of values which has been found to give good resultsis as follows:

ss 1 meg. l34 0.1 meg. 87-.000075 mid. l36-.000075 rnfd. 58-1 meg, 120.lmeg. 731 LB 4 li33,0(l0 ohm ll920 mfd. 123% meg. l24% meg. 122l,000 ohm750,000 ohm %7F7 9975,000 ohm fi th-.0005 mfd.

92VR75 93-VR-105 132-250 ohm 12.t-20 n fd.

E35.000O75 mfd. 1375O meg.

The current drawn by even a large Geiger-Muller tube is exceedinglysmall, being of the order of A of a microampere.

While the specific embodiment has been drawn chiefly to a Geiger Mullersystem, it will be understood that most of the members can be applied toother systems such as those utilizing ionization chambers. Thus, theconstruction of the tube and the mode of its use may be applied to anionization chamber apparatus. Further, the high voltage may be appliedin the same way. The transmitting circuits and the association of thecircuit elements may also be utilized in ionization chamber assemblages.

The scope of the invention is indicated by the appended claims.

We claim:

1. In an apparatus for the measurement of radioactivity within a borehole, a cartridge adapted to be lowered Within the bore hole, and tocontain elements responsive to radioactivity therein, said cartridgehaving within it a radioactive sensitive element to be supplied with avoltage whose value depends upon the temperature, means to receive avalve from the surface of the earth, and to apply a proper operatingvoltage for the said radioactive sensitive element, the aforesaid meanshaving combined therewith a temperature compensated element adapted tovary the voltage supplied to the said radioactive element in proportionto the temperature in said bore hole.

2. The method of detecting radiation wherein an instrument capable ofdetecting radiations encounters regions of varying temperature whichcomprises impressing a direct current potential on an electrode of aradiation detector of the counter type to yield current pulses, as aresult of radiations, and varying the potential pressed on theelectrodes of said detector directly as a function of the temperatureencountered by said detector, thereby maintainins the detector ineffective operating condition.

well logging appara is in which a radioactivity mber for rneast ingpenetrative radiation through a ell, said ratioactivity responsivechamber being capable of generating electrical current of a fraction 0:.a microampere that is a function magnitude of re order of a billion ohmsconnected in the output c'; it of said radioactivity responsive chambet,an instrument for measu ing the voltage drop across said resistor andtemperature controlling means for maintaining said resistor at apredetermined temperature during the passage of the instrument throughthe hole.

4. well logging apparatus, the combination of a cartridge adapted to belowered into a well, a radioactivity responsive device in the cartridge,said device including element to be supplied with voltage, electricoscillator means including a tuned electrical circuit in the cartridge,means for rectifying the output of said oscillator means, filter meansfor smoothing the output of said rectifier means, circuit means forsupplying smoothed output to said element of the radioacti' 'tyresponsive device, and thermally responsive '"e means connected to saidtuned circuit for varying at least one property of said tuned circuit asa function of the temperature in the Well, thereby also varying thevoltage applied to said element of the radioactivity rcsponsive deviceas a function of the temperature in the well.

in an apparatus measuring radioactivity within a deep narrow bore hole,a cartridge capable of being lowered to a desired depth within a borehole, and to contain elements responsive to a radioactivity therein,power supply means in said cartridge adapted to receive a voltage fromthe surface of the earth, a radioactivity detecting means vvithin saidcartridge supplied with a proper operating voltage from said powersupply means, means to transmit the responses from said radioactivitydetecting means to the surface of the earth, means to receive no saidresponses, the responses of said detecting means being dependent notonly upon the radioactivity in the vicinity of said cartridge but uponthe operating voltage and temperature of said detecting means, meansdefining an evacuated insulating space at least partially surroundingsaid detecting means and serving to prevent substantial alteration inresponse thereof due to the temperature variations in said bore hole,and means adapted to regulate the voltage applied to said detectingmeans by said power supply means, whereby the response of said detectingmeans while traversing said bore hole is less sub ect to undesiredvariation.

References Cited in the file of this patent UNITED STATES PATENTS

1. IN AN APPARATUS FOR THE MEASUREMENT OF RADIOACTIVITY WITHIN A BOREHOLE, A CARTRIDGE ADAPTED TO BE LOWERED WITHIN THE BORE HOLE, AND TOCONTAIN ELEMENTS RESPONSIVE TO RADIOACTIVITY THEREIN, SAID CARTRIDGEHAVING WITHIN IT A RADIOACTIVE SENSITIVE ELEMENT TO BE SUPPLIED WITH AVOLTAGE WHOSE VALUE DEPENDS UPON THE TEMPERATURE, MEANS TO REVEIVE AVOLTAGE FROM THE SURFACE OF THE EARTH AND TO APPLY A PROPER OPERATINGVOLTAGE FOR THE SAID RADIOACTIVE SENSITIVE CLEMENT, THE AFORESAID MEANSHAVING COMBINED THEREWITH A TEMPERATURE COMPENSATED ELEMENT ADAPTED TOVARY THE VOLTAGE SUPPLIED TO THE SAID RADIOACTIVE ELEMENT IN PROPORTIONTO THE TEMPERATURE IN SAID BERE HOLE.